As London's transport system faces its first major test of the Olympics, organisers say security has not been compromised by the loss of keys at Wembley Stadium.
Olympic organisers expect 1 million extra visitors in London to make around 3 million journeys on public transport, on top of the city's daily commuters.
People are being warned to prepare for delays as transport restrictions are put in place to cope with extra passenger numbers.
Monday is the first major day of Olympic competition on a weekday.
Transport for London (TfL) says London Bridge, Bank and Canary Wharf are expected to be exceptionally busy because of the equestrian at Greenwich.
TfL is also urging motorists to check road closures before travelling.
A section of a main road route into London had to close after a serious accident on the London-bound section of the M4 between junction 5 at Langley in Berkshire and junction 4b near Heathrow airport in west London.
Elsewhere officers searching Wembley ahead of the start of events misplaced a set of keys, prompting an investigation by Scotland Yard.
Games organisers last night insisted that security had not been compromised by the incident.
They stressed that the keys were internal and that the relevant locks have now been changed.
Scotland Yard is not treating the incident as criminal.
A spokeswoman for the Metropolitan police said: "On the morning of Tuesday 24 July officers on Olympic police operations at Wembley stadium reported that internal security keys, being used by them as part of searches, were missing."
She said fellow officers attended the scene to help look for the keys but nothing was found.
"Detectives also attended to ascertain if there was any evidence of criminal offences," the spokeswoman added.
"There's none at this time."
The spokeswoman said that Wembley officials were made aware of the incident and added: "There is absolutely no security concern in relation to the stadium as measures were taken immediately to secure all key areas of the venue."
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Fans hoping to buy snacks as they watched Great Britain's men take on United Arab Emirates at Wembley were told they could not pay by Visa, the only credit or debit card accepted at London 2012 venues, and must use cash.
Max Gore, 27, from Shepherd's Bush, west London, said: "The queues were ridiculous.
"I didn't have any cash and I couldn't pay on card."
Mr Gore added: "The problem went on for the whole evening. There were two games and they couldn't get it fixed."
Thousands of fans attended the stadium today to watch Senegal play Uruguay before Great Britain's match.
A Visa spokesman said: "We understand that Wembley's systems failed and therefore they were only accepting cash at the food and beverage kiosks.
"This cash only decision was made by Wembley management and not Visa.
"We are working with the Wembley team to help them fix this as soon as possible."